1. Field of the Invention
The present invention relates to an apparatus for recording an image on a recording surface with a multibeam, i.e., a plurality of laser beams.
2. Description of the Background Art
FIG. 1 shows a first example of a conventional apparatus for recording an image with a multibeam (laser beams). The first conventional apparatus comprises a plurality of laser source units 1 mounted at equal intervals on a holding member A having a spherical surface. For convenience of description, FIG. 1 shows only one of the laser source units 1.
The laser source unit comprises a semiconductor laser 1a and a collimator lens 1b. The collimator lens 1b shapes a laser beam from the semiconductor laser 1a into a collimated laser beam LB and directs the laser beam LB to an afocal optical system AL1 such that the principal ray of the laser beam LB passes through the front principal point of the lens L1, the afocal optical system AL1 being formed by the lens L1 and another lens L2. The laser beam LB is expanded by the afocal optical system AL1 so as to have an enlarged beam diameter, and then focused onto a recording surface 3 through a lens L3.
Laser beams from the other laser source units 1 are focused onto the recording surface 3 in a similar manner. Thus, focused with the laser beams, a plurality of beam spots are formed at a time on the recording surface 3.
FIG. 2 shows a second example of a conventional apparatus for recording an image with a multibeam. The second conventional apparatus comprises a plurality of laser source units 1 disposed so as to be equally distant from each other, an afocal optical system AL1 formed by lenses L1 and L2 and an afocal optical system AL2 formed by lenses L3 and L4. FIG. 2 also shows only one of the laser source units 1. In FIG. 2, a laser beam LB from the laser source unit 1 enters the afocal optical system AL1 such that the principal ray thereof advances parallel to an optical axis Z. The laser beam LB is thereafter focused onto a recording surface 3 through the afocal optical systems AL1 and AL2.
Laser beams from the other laser source units 1 not shown in FIG. 2 are focused onto the recording surface 3 in a similar manner. Thus, a plurality of beam spots are formed at a time on the recording surface 3.
Due to the structures as described above, the first and the second conventional recording apparatuses must include an increased size lens to form a greater number of beam spots on the recording surface 3, i.e., to increase channel numbers. In the second conventional apparatus, for instance, the number of the laser source units 1 must be increased in a direction perpendicular to the optical axis Z in order to ensure a larger numbers of the channels (FIG. 2). To accommodate to the increase in the number of the laser source units 1, the lens L1 must be larger. When the lens L1 has an enlarged size, aberration caused by the same intensifies and manufacturing costs thereof swell up. In addition, the size of the recording apparatus must be also large enough to house the enlarged lens L1.
The first conventional apparatus of FIG. 1 is faced with the same problems because the lens L2 must be enlarged to increase the numbers of the channels. In addition, the first conventional apparatus has still another problem. Since the laser source units 1 in the equidistant relation are mounted on the holding member A having the spherical surface, distances between beam spots on the recording surface 3 will not be equal. As a result, raster scanning using the first conventional apparatus will not produce an intended image.